In wireless access networks such as UMTS Terrestrial Radio Access Network (UTRAN) and Evolved (E-UTRAN), the amount of data transmitted in a Transmission Time Interval (TTI) may vary considerably. This variation can depend on aspects such as transmitter buffer status, link quality, and scheduling strategy.
The amount of data transmitted in a TTI is typically reflected by the size of a Transport Block (TB), Transport Format (TF), Transport Format Combination (TFC), or a similar attribute. This “block” defines how many information bits that are transmitted in a TTI.
Typically, the transmitter performs the transport format selection, and indicates the selected transport format through out-band signaling. This is the case in UTRAN where in the following applies:                Downlink: The High-Speed Downlink Shared Channel (HS-DSCH) downlink Transport Format is derived from the Transport Format Resource Indicator (TFRI) carried on the High Speed Shared Control Channel (HS-SCCH) channel,        Uplink: The Enhanced Dedicated Channel (E-DCH) uplink Transport Format is indicated out-band with the E-DCH Transport Format Combination Indicator (E-TFCI) on the Enhanced Dedicated Physical Control Channel (E-DPCCH) channel.        
This is described in e.g. 3GPP standard 25.321.
Alternatively, the receiver may select the Transport Format. This is the case in the Long Term Evolution (LTE) uplink, where the evolved NodeB (eNB) selects the transport format that the User Equipment (UE) shall use. In this case, the indication goes from the receiver to the transmitter on the Physical Downlink Control Channel (PDCCH) channel prior to the transmission of the data-block. Note that in LTE (E-UTRAN), the transport format selection is made by the eNB for both uplink and downlink transmissions.
A third alternative is denoted blind detection which is the mechanism when no format indication is transmitted in parallel or prior to the data transmission. To find the correct transport format, the receiver needs to blindly decode multiple formats. To reduce the computational complexity, the applicable formats are often reduced to a few. One benefit of blind decoding without transport format indications is that out-band signaling can be reduced. This solution is available e.g. for UTRAN downlink “HS-SCCH-less transmission”.
UTRAN System
Rel-6 of the Enhanced Uplink concept (Enhanced uplink), E-UL, of E-DCH, where E-DCH stands for the Enhanced Dedicated Transport Channel) supports peak bit-rates up to 5.7 Mbps. Rel-7 has recently been updated with higher order modulation (16 Quadrature Amplitude Modulation, QAM) providing peak-rates beyond 10 Mbps.
The E-TFCI indication is carried by 7 bits on the E-DPCCH. This means that out-band signaling can indicate 128 different Transport Block sizes. Normative tables for E-TFCI values and corresponding Transport Block sizes can be found in Annex B of 3GPP standard 25.321. The most recent release (Rel-7) of the Media Access Control (MAC) specification has been updated with several new tables to support higher peak data rates and to minimize the amount of padding:
The quantization of available Transport Block (TB) sizes (128 for E-DCH) implies that not all different TB sizes are available. This means that, unless there is a perfect match of the buffer size and/or size of higher layer Packet Data Units (PDUs), padding has to be used to fill the remaining bits of the TB. If the E-TFCI has to span a large range of sizes starting from small E-DCH Transport Format Combinations (E-TFCs) (resulting in only a couple of kbps) up to large E-TFCs (resulting in several Mbps), it means that the step-sizes in the table have to be quite large. The support of higher bit-rates will increase the amount of padding, since the number of E-TFCI:s that need to span the whole operating region remain the same.
The Rel-7 MAC specification (25.321) supports several E-TFCI tables. Some of the tables have been optimized to minimize padding for the most common (fixed) Radio Link Control (RLC) Packet Data Unit (PDU) size (336 bits), while other tables have been optimized to minimize the padding with respect to other criteria. For example other tables can be optimized for the relative amount of padding for arbitrary size upper-layer payload, or inclusion of in-band signaling messages such as the Scheduling Information message.
E-UTRAN System
The E-UTRAN supports an out-band control channel, PDCCH, upon which both Uplink (UL) and Downlink (DL) transport formats will be indicated. One major difference to UTRAN concerns the uplink: in E-UTRAN it is the eNB that selects the transport format also for the uplink. Thus, the User Equipment (UE) will have to obey the format selection indicated on the PDCCH.
Furthermore scheduling may be performed by “Persistent Scheduling” or “Semi-Persistent Scheduling”. With (semi)persistent scheduling, the desire is to reduce the amount of traffic on the PDCCH control channel by issuing grants that have a validity spanning over several TTIs. These multiple TTIs for which the persistent grant is valid could occur periodically, e.g. every 20 ms. Such a solution can be particularly useful e.g. for Voice over IP (VoIP) traffic. Alternatively, a persistent grant can span several consecutive TTIs.
There are different proposed solutions for control of persistent and semi-persistent grants. One solution is to use a dedicated information bit on PDCCH to indicate if a grant is persistent or not. However, this solution can be considered quite costly, as the bit would be reserved also when no persistent scheduling is used, see R2-080088, “Configuration of semi-persistent scheduling”. Source: Ericsson.
Since persistent scheduling is considered as an add-on to regular scheduling, this approach is quite costly. Alternative solutions include control using inband mechanisms by Media Access Control (MAC) or Radio Resource Control (RRC). However, these upper-layer methods are subject to delays, as the MAC control elements or RRC control signals are subject to Hybrid Automatic-Repeat-Request (HARQ) (re-)transmissions.
Yet another E-UTRAN concept is denoted “HARQ Autonomous Retransmissions”, or “TTI Bundling”, see R2-072630, “HARQ operation in case of UL power limitation”, Source: Ericsson. In this TTI Bundling concept, several HARQ re-transmissions of the same payload is issued in consecutive TTIs without waiting for HARQ feedback from the receiver. The desire with this concept is to improve coverage without introducing excessive HARQ re-transmission delays in cases when many HARQ re-transmissions are needed to achieve successful receiver decoding of a Transport Block.
As for the persistent scheduling, the TTI Bundling solution is associated with a control problem. Hence, there is a need for a solution to indicate if a transmission is a regular scheduled transmission, or if re-transmissions of the Transport Block should be issued in subsequent TTIs without waiting for HARQ feedback.
High costs in terms of radio resources of PDCCH hinder solutions where a feature, here exemplified by Persistent Scheduling and TTI Bundling, occupies dedicated bits on the PDCCH. Therefore, there is a need for a cost efficient solution to control such features which occupies dedicated bits on the PDCCH in E-UTRAN.
Several problems exist. For UTRAN, the uplink in UTRAN Rel-8 is currently being updated with Improved Layer 2 (L2) including Flexible RLC PDUs and MAC segmentation. It has recently been identified that the Transport Format tables available in MAC Rel-7 may not be optimal. With Flexible RLC PDUs, where RLC PDUs can take any suitable size, it has been identified that new E-DCH transport block sizes would be desirable. In particular, at least one new or modified small transport block is needed to improve coverage and to reduce padding. A problem is that in most cases, all (128) E-TFCI indications in the MAC E-TFCI tables are occupied. If new formats are introduced, or if Transport Format mapping to E-TFCIs are changed, then it is necessary to introduce new E-TFCI tables for Rel-8, in order to maintain backwards compatibility. As a consequence, Rel-7 already includes several E-TFCI tables, and more may be added.
It can be noted that in Rel-99, the Transport Format Combinations are configured using upper layer signaling including Radio Resource Control (RRC), Node B Application Part (NBAP) and Radio Network Subsystem Application Part (RNSAP). However, this leads to costly signaling, and for HS-DSCH and E-DCH this solution is not possible to implement in practice due to the fact that the vast amount of Transport Formats (E-TFCIs and (HS-TFRIs) are needed both in the UE and in the Node B. Therefore, the solution with tables specified in MAC was adopted, and upper layer signaling only indicates which table that should be used.
Another problem that relates both to UTRAN and E-UTRAN is that different applications, such as Multi Media Telephony (MMTel) including Voice over Internet Protocol (VoIP) can have very specific packet size distributions. This is for example the case for particular voice encoders that mainly generate packets of a few discrete sizes. Different encoders result in different (but known) packet-size distributions.
To minimize padding, it would be desirable to tailor the transport formats such that the Transport Blocks available would suit the most frequently used application packets. However, it is not practical to introduce new E-TFCI tables for every new application or codec. As new applications (with new packet-size distributions) are introduced in the future, it would be desirable to have flexibility in MAC, such that the most suitable Transport Formats could be introduced without specifying new (E-TFCI) tables in the MAC specifications. This would be particularly attractive in Rel-8 when RLC supports flexible sizes both in uplink and downlink.
In addition, as described above, the Rel-7 MAC specification has recently been updated with new E-TFCI tables to support peak rates beyond 10 Mbps. Since the number of E-TFCI bits has not been expanded in Rel-7, it means that the quantization in the new tables is less flexible than the older tables, because the available E-TFCI code-points have to span a larger space. This means that the amount of padding will increase in Rel-7.
Yet another problem which relates to E-UTRAN is that, as is already described above, there is a need for a cost-efficient solution for indicating if a grant issued and signaled on PDCCH is valid only for a single TTI (regular scheduling), or if the validity of the grant is Persistent, i.e. if the validity of the grant spans over several TTIs. Persistent scheduling where the grant is valid periodically is shown in FIG. 1.
An additional problem that is related to E-UTRAN systems is that, as is already described above, there is a need for a cost-efficient solution for indicating if a grant issued and signaled on PDCCH is valid only for a single TTI (regular scheduling), or if the grant is a grant for “Bundled TTIs”, (Autonomous Re-transmissions), where the transmitter should issue HARQ re-transmissions without waiting for HARQ feedback. Typically, these autonomous re-transmissions would be issued in subsequent TTIs, see FIG. 2.
The E-TFCI indication is carried by 7 bits on the E-DPCCH. This means that that the out-band signaling can indicate 128 different Transport Block sizes. Normative tables for E-TFCI values and corresponding Transport Block sizes can be found in Annex B of 25.321. The data transmission (i.e. the actual Transport Block) is transmitted on the E-DPDCH channel(s). The transmission power of the E-DPDCH is set with a power offset relative to DPCCH (see 25.214 for details), where DPCCH is power-controlled (through fast power control) from the Node B. Each Transport Format is thus associated with a specific power offset, such that a large E-TFC is transmitted with higher power compared to smaller E-TFCs. As a tool for Quality of Service (QoS) differentiation, it is further possible to have specific offsets for different MAC-d flows, such that the offset is different for an E-TFC depending on what payload it carries. The power offsets for E-DPDCH and E-DPCCH relative to DPCCH are schematically illustrated in 3. The DPCCH is power controlled by the Node B such that it is received at the Node B with a certain Signal-to-Interference Ratio (SIR). E-DPCCH is sent with a power offset relative to DPCCH. This is also the case for E-DPDCH, but the actual offset depends on the choice of transport format and the HARQ profile used for the transmission, see also the 3GPP standard 25.321. A large format is typically sent with a large offset, while a small format is sent with a small offset.
The Rel-7 MAC specification has recently been updated with new E-TFCI tables to support peak rates beyond 10 Mbps. Since the number of E-TFCI bits has not been expanded in Rel-7, it means that the quantization in the new tables is larger than the older tables, because the available E-TFCI code-points have to span a larger space. This means that the amount of padding is likely to increase when these new tables are taken into use. As a result from this Rel-7 terminals will be less efficient in this respect, since more padding will be transmitted on average.
A solution to this padding problem could be to expand the number out-band bits carrying E-TFCI on E-DPCCH. However, this approach introduces additional out-band overhead. There are also limited means to expand the number of bits used for E-TFCI without a considerable change of the E-DPCCH physical channel.
With increasing bit-rates, it would therefore be desirable to find a method that avoids excessive padding together with a low overhead on the out-band signaling, i.e. that only a few bits for TFC indications are used.
Hence, there exist a need for a method and a system that is able to improve existing transmission schemes for cellular radio systems and which enable more efficient transmission of transport blocks.